1. Technical Field
The present disclosure relates to an image display device which is preferable for displaying virtual images via a half mirror, for example.
2. Description of the Related Art
In recent years, an image display device which causes a virtual image optical system formed by combining surface reflection (or half mirror) on a glass surface and a free-form surface mirror to display video images on a space is being developed.
Particularly, application to an image display device mounted on a movable body such as an automobile, which is referred to as a head-up display, is expected. For example, a head-up display mounted on an automobile projects light modulated based on image information, to a windshield (front glass), and reflection light of the light is irradiated on the driver's eyes. Consequently, the driver can see a virtual image ahead of the windshield. For example, a vehicle speed, an outdoor temperature and the like are displayed as a virtual image. Recently, it also has been studied to display a navigation image and an image which calls for attention to existing passengers as a virtual image for a driver.
Laser light sources such as semiconductor lasers are used as light sources for the head-up display. According to this configuration, laser light is modulated according to a video signal, and laser light scans the screen. The screen diffuses the laser light, and widens a light area to be irradiated on the driver's eyes. Consequently, even when the driver moves the head more or less, the eyes do not go out of an irradiation area, so that the driver can stably see images (virtual images) well.
By using the laser light sources, it is possible to make small an optical system that makes laser beams scan on the screen. Further, a combination with a virtual image optical system having a larger optical magnification makes it possible to display large virtual images even in the case of a smaller main body dimension. Hence, it is expected to increase the number of models of the vehicle on which head-up displays are mounted.
However, there is a problem that using the laser light sources as light sources generates interference noise, i.e., so-called speckle noise due to coherence of laser light, and a display image becomes glaring and is hardly viewed.
Many technical experts have devised measures for reducing the speckle noise and proposed some methods for the speckle noise problem so far.
For example, Unexamined Japanese Patent Publication No. 2008-26616 discloses a speckle noise reducing method for applying a polarization rotation coating to microlens arrays and making a lens dimension equal to or more than a beam diameter.
Further, International Publication No. 2012/117495 discloses a configuration where a screen is configured by disposing two microlens arrays of hexagonal lattice shapes to oppose to each other and shifting an angle of the second microlens array to reduce speckle noise.
Unexamined Japanese Patent Publication No. 2013-64985 discloses a configuration where speckle noise is reduced by determining a lens pitch such that a pitch between a plurality of optical elements aligned in a lattice shape is a pupil diameter of an eyeball or less.
International Publication No. 2009/019973 proposes a technique for canceling speckle noise by screen vibration, and discloses that speckle noise is reduced when a spot size satisfies a relationship of spot size<screen movement amount<π×spot size 2÷(2×laser wavelength).